In recent years, a noncontact information medium such as an IC card has been put into practical use. Such a medium can supply power via radio waves by using a mutual induction phenomenon of coils and can transmit and receive data.
An IC card as an example of a noncontact information medium is broadly categorized into a close proximity type, a proximity type, a vicinity type, and so on based on communication distances between the IC card and a reader/writer, which transmits and receives radio waves to and from the IC card. Standardization has been prepared for each of the types.
Particularly a proximity-type IC card, which can be used at a distance of about 0 to 10 cm from a reader/writer, may have quite extensive application. For example, when the IC card is used for a commutation ticket and the like, without taking out the IC card from a commutation-ticket holder at a ticket gate of a station, the opening and closing of the ticket gate can be controlled according to exchange of information with a reader/writer in a noncontact state.
However, smaller and lighter IC cards are important for the more extensive application. The more extensive the application of IC cards is, the less care will be taken for the handling of them. Thus, in view of resistance to a breakdown from rough handling, a semiconductor integrated circuit having a complicated circuit in a small area has been normally mounted in a noncontact information medium such as an IC card.
Referring to FIGS. 7 to 10, the following will discuss the technique of a conventional noncontact IC card with a semiconductor integrated circuit embedded in it.
FIG. 7 shows the conventional noncontact IC card and reader/writer.
A noncontact IC card 60 is constituted by a noncontact IC card integrated circuit 61 (hereinafter, “integrated circuit” will be referred to as “LSI”), an antenna coil 62, and a tuning capacitor 63. The LSI 61 is constituted by an analog circuit 70, a logic circuit 71, a memory circuit 72, and so on.
To pads 90 and 91 of the LSI 61, the antenna coil 62 is connected for transmitting and receiving radio waves 66 to and from an antenna coil 65, which is connected to a reader/writer 64. The tuning capacitor 63 is connected to the antenna coil 62. The antenna coil 62 receives radio waves from the reader/writer 64, and alternating voltage is produced across the antenna coil 62 (between the pad 90 and the pad 91). The alternating voltage produced across the antenna coil 62 is applied to the analog circuit 70.
The analog circuit 70 is constituted by a rectifier circuit 80, a power supply circuit 81, a clock generating circuit 82, a demodulator circuit 83, a modulator circuit 84, and so on. In the analog circuit 70, power supply voltage L for operating the logic circuit 71 and power supply voltage H for operating the memory circuit 72 are produced by the rectifier circuit 80 and the power supply circuit 81.
The clock generating circuit 82 generates a clock by using alternating voltage, which is produced across the antenna coil 62, as an input signal. The above clock CLK operates the digital circuit 71 and the memory circuit 72.
Data transmitted and received between the noncontact IC card 60 and the reader/writer 64 is transmitted and received while being superimposed on radio waves (the above alternating voltage). When the noncontact IC card 60 receives data from the reader/writer 64, the IC card performs demodulation in the demodulator circuit 83 to obtain a demodulation signal (RXDATA). When the noncontact IC card 60 transmits data to the reader/writer 64, a transmitted signal (TXDATA) is modulated in the modulator circuit 84.
In this manner, data transmitted and received between the IC card and the reader/writer 64 is interpreted in the logic circuit 71, the data is stored in the memory circuit 72 after addresses and data are specified, and the data is read after an address is specified.
Here, referring to FIGS. 8 and 9, the following will discuss the rectifier circuit 80, the power supply circuit 81, and the demodulator circuit 83 in the analog circuit 70.
As shown in FIG. 8, alternating voltage produced across the antenna coil 62 is directly inputted to the rectifier circuit 80 via the pads 90 and 91. The rectifier circuit 80 acts as a voltage doubler rectifier circuit composed of diodes 100 and 101.
The power supply circuit 81 is constituted by a shunt circuit 110 and smoothing capacitors 111 and 112, and power supply voltage H is clamped to a predetermined voltage by the shunt circuit 110.
The operating principle of the rectifier circuit 80 is shown in FIG. 9.
FIG. 9(A) shows alternating voltage 120 (voltage relative to the pad 90: voltage 121) produced across the antenna coil 62 when data is transmitted from the reader/writer 64. The data has been subjected to ASK modulation at a carrier frequency of 13.56 MHz, which is used for communication of the noncontact IC card.
The following will discuss the case in which the reader/writer 64 transmits data by encoding NRZ. The data is obtained by performing ASK modulation on digital data. Namely, when the reader/writer 64 transmits “H” data, the alternating voltage 120 across the antenna coil is set at a high level. When the reader/writer 64 transmits “L” data, the alternating voltage 120 across the antenna coil is set at a low level.
First, the following will discuss how power supply voltage is produced by the alternating voltage 120 produced across the antenna coil 62. Here, it is easier to understand when a terminal voltage of the pad 90 is set at a reference voltage 121.
Negative component voltage (VSS) 122 is produced by the diode 100 of the rectifier circuit 80. The power supply voltage L is smoothed by the smoothing capacitor 111. Further, power supply voltage H (level 123 of FIG. 9) is produced from positive component voltage by the diode 101 of the rectifier circuit 80, and the power supply voltage H is smoothed by the smoothing capacitor 112.
In the IC card, since a distance from the reader/writer 64 is changed, the alternating voltage 120 produced across the antenna coil 62 (between the pad 90 and the pad 91) is changed even when the radio waves 66 transmitted from the reader/writer 64 are constant.
Namely, when the reader/writer 64 and the IC card are in contact with each other, the alternating voltage 120 increases in level. Further, when the reader/writer 64 is away from the IC card, the alternating voltage 120 decreases in level.
In the case of ISO14443 (proximity-type noncontact IC card: standard of a communication distance of about 10 cm), which is an international standard of noncontact IC cards, depending upon the shapes of the antenna coil 66 in the reader/writer 64 and the antenna coil in the IC card, the intensity of the radio waves 66 received by the IC card is changed by five to ten times in a close state as compared with a distance of 10 cm. Assuming that the power consumption of the LSI 61 is constant regardless of voltage, power supply voltage is changed by five to ten times.
Namely, in the case of a distance of 10 cm between the IC card and the reader/writer 64, when the power supply voltage H is about 4V, by shortening the distance between the IC card and the reader/writer 64 close to 0 cm, the power supply voltage H rises to 20 V or more so as to damage the built-in LSI 61.
Thus, power supply voltage is clamped by the shunt circuit 110 to increase consumed current of the LSI 61 in appearance.
FIG. 10 shows the voltage and current characteristics of the shunt circuit 110.
When a final-stage transistor for determining current and voltage characteristics is composed of a MOS transistor, the shunt circuit 110 conducts current at a voltage more than a predetermined threshold voltage according to a square function of voltage. Further, when the final-stage transistor is composed of a bipolar transistor, the shunt circuit 110 conducts current at a voltage more than a predetermined threshold voltage according to an exponential function. In the case of FIG. 10, the shunt circuit 110 hardly consumes current when the power supply voltage H is 4V, and the shunt circuit 110 consumes current of 10 mA when the power supply voltage H is 5V.
Namely, in a state in which large current is applied to the shunt circuit 110, a change in power supply voltage is smaller even when current is changed.
FIG. 9(B) shows the state of power supply voltage at a long distance that communication is possible.
When the reader/writer 64 transmits “H” data, the power supply voltage H is set at a high level (about 5V), and when the reader/writer 64 transmits “L” data, the power supply voltage H is set at a low level (about 4V).
FIG. 9(C) shows the state of the power supply voltage at a short distance that communication is possible.
When the reader/writer 64 transmits “H” data, the power supply voltage H is set at a high level (about 5.5V), and when the reader/writer 64 transmits “L” data, the power supply voltage H is set at a low level (about 5.3V).
Namely, the shunt circuit 110 supplies large power supply current when the power supply voltage H is increased (FIG. 10). Thus, when the power supply voltage H is increased, the shunt circuit 110 has greater capability of conducting current, resulting in smaller change in power supply voltage. Therefore, a change in power supply voltage H is reduced.
In the conventional analog circuit 70, the demodulator circuit 83 receives power supply voltage and detects a rate of change in power supply voltage. Thus, a change in power supply is reduced at a short distance that communication is possible, so that it becomes difficult to produce a demodulating signal (RXDATA).